Use your normal detergent- I use Tide (it's nice too because the kind I use is free of perfumes and dyes!) You shouldn't use any softener as it decreases the "wickiness" or absorbancy of the fleece. For the layers, under the towels you can have a number of things: newspaper, puppy pads, plastic, etc. If you have enough towels underneath the fleece, the pee shouldn't wick all the way through to the last layer though.
There is a sticky all about fleece on the bedding section, it really helped me! ...
http://www.guineapigcages.com/forum/...ect-study.html